Alternate domain for one page only

My user wishes to use a different domain name for a single “page” of her WP.

domainA.tld has WP installed right at the document root instead of a subdirectory
/home/user/public_html/
(foo referenced below is not a physical folder, it’s a category)

domainB.tld is currently sharing this document root (add-on domain in cpanel)

i.e. Whenever /foo/bar/ is called up, instead of this:
h tt p : // domainA.tld/foo/bar/

show simply this URL in the address bar:
h t t p : // domainB.tld

Also, the generated hyperlinks to /foo/bar/ should use domainB:
domainB.tld or domainB.tld/foo/bar/ would be acceptable

Is there an existing plugin (multisites overkill for this?) or a combination of mod_rewrite and quick code hack that can do this?
I did try mod_rewrite alone, can’t get it right. Not familiar enough with WP to know how it may be interacting/generating its URLs.

CPanel/Apache 2.x and I am comfortable editing PHP files.

Plan B is to “view source” and turn it into a static page in a separate document root for domainB. Would need to be updated whenever she alters categories/menus/themes or the content of this page itself, so this approach is less than ideal, especially since part of the reason it’s not on its own in the first place is to help facilitate maintenance.